There are two motions that take place when a vehicle turns. A first motion is a turning motion called yaw motion. Yaw motion takes place as the vehicle spins around an imaginary axis that is vertical to the ground. A second motion is a lateral sliding motion called side-slip motion. Depending on a speed of the vehicle, side-slip motion occurs in the same direction as a turn or in an opposite direction of the turn.
A command interpreter in a stability control system of a vehicle determines a desired yaw rate and a desired side-slip velocity for the vehicle based on speed and a position of a steering wheel. The desired yaw rate and side-slip velocity correspond with values that typically exist when a vehicle is traveling on a dry and clean surface. When an actual yaw rate and/or side-slip velocity of the vehicle significantly surpasses a desired value, a driver typically feels a loss of control of the vehicle. In this case, it is likely road conditions necessitate vehicle stability enhancement.
In one approach, a stability control system compares actual yaw rate and/or side-slip velocity measurements with desired yaw rate and/or side-slip velocity values. The stability control system takes corrective action when the actual measurements surpass the desired values by predetermined thresholds. For example, the stability control system may instruct one or more brake actuators to create a brake pressure difference across one or more axles of the vehicle to create a yaw moment that stabilizes the vehicle. The stability control system may also instruct a rear-wheel steering actuator to turn a set of rear wheels of the vehicle to create the yaw moment.
Commercially available yaw rate sensors measure a yaw rate of a vehicle. Side-slip velocity sensors measure a side-slip velocity of a vehicle. However, side-slip velocity sensors are very expensive. Side-slip velocity may be determined based on a side-slip acceleration of a vehicle. For example, side-slip acceleration may be estimated based on a lateral acceleration, yaw rate, and speed of a vehicle. Ideally, side-slip velocity may be obtained by integrating side-slip acceleration. However, sensor bias always exists in sensors such as yaw rate sensors and lateral accelerometers. Therefore, the integration drifts because the unwanted bias signal is also integrated.
In one approach, a dynamic observer captures an estimated state of dynamics for a vehicle. An estimated side-slip velocity is generated based on the estimated state of dynamics. However, the estimation is based on vehicle cornering compliances, which are variable vehicle parameters. Cornering compliances vary greatly and depend on the type of surface that the vehicle is operating on. Therefore, the estimation is not accurate.
In another approach, global positioning system (GPS) sensors detect a position of a vehicle. Side-slip velocity is estimated based on data from the GPS sensors. However, GPS sensors are very expensive.